The invention relates to computed numerical control of a drilling machine by which drilling of very minute diameter may be performed.
This kind of the drilling machines are mainly used for making very small holes in, e.g., a precision-finished metal die for escaping the air, being about 0.1 mm diameter, in which the air could enter such small hole but a molten substance could not go into the hole, or a special injection nozzle of combustion, connecting point of glass fiber to glass fiber for light communication, being 0.125 mm diameter for required 0.126 mm diameter, surgical operational device, and others.
There are conventional ways of drilling processes of this kind, which are roughly divided into
(1) a process performed in dependence upon a step feed mechanism utilizing a closed loop control system, which is housed in a precision sizing device, and
(2) a process performed in dependence upon a step feed mechanism utilizing an open loop control system.
In the process (1), if a required drilling diameter were less than 0.5 mm, the drill would be easily broken and it is extremely difficult to make perforation of around 0.1 mm diameter.
In known embodiments of the step feed mechanisms using the open loop control system (2), one of them uses an air timer to open and close a switch pneumatically. Another utilizes a rotary cam to open and close a switch by a cam. The other employs a step motor. However under the present circumstances excepting the step feed mechanism, they would be unsuitable for making the small perforation of minute diameter.
The control of the drill process in general comprises repetition of the process of forming a cut origin on the surface of an object to be processed, moving down the drill from the cut origin by a certain feed amount, returning it to the cut origin, farther moving it down and again returning to the origin.
An explanation will be made to the step feed mechanism using the step motor now reduced to practice. A control device thereof is based on a digital control using IC and transistor. However it is difficult to provide complicated control of the feeding amount by fine control of the step motor. For performing the drilling of the small diameter, said repetition is carried out between the cut origin and the processing point to make the processing deeper at the low processing speed. If the hole diameter were required less than 0.1 mm, the drilling efficiency would be extremely low. In the actual requirements, the feed speed could not be reduced less than a certain limit. Therefore the drilling process for holes of less than 0.3 mm diameter is regarded as impossible.